Method and apparatus to prepare a tribasic propellat charge powder

ABSTRACT

The invention relates to a method of and an apparatus for producing tribasic propellant charge powders. An operationally safe and continuous course of the production process is obtained according to the invention by first preparing a fully gelatinized dibasic intermediate product of nitrocellulose and a blasting or explosive oil, without solvent, in a shearing mill (1). The intermediate product is granulated and supplied to an extruder (4) into which the third crystalline energy carrier and solvents are introduced. There the pulverous mixture is homogenized and extruded to form powder strands which are moist with solvent.

The invention relates to a method of making a tribasic propellant chargepowder of nitrocellulose, blasting or explosive oil, and a thirdcrystalline energy carrier, as well as additives, making use of asolvent. The invention also relates to an apparatus for carrying outthat method.

The tribasic propellant charge powders are known also as "cold" powders.They have the advantage of better barrel protection, i.e. prevention oferosion than the conventional "hot" powders which do not contain a thirdcrystalline energy carrier.

In making propellant charge powders with an explosive oil, thisexplosive oil acts as a gelatinizing agent for the nitrocellulose. Inthe case of tribasic propellant charge powders the quantity of blastingoil often is not sufficient for the complete gelatinizing of thenitrocellulose and for uniform and homogeneous binding of the thirdcrystalline energy carrier in the powder mixture, particulary if thecrystalline energy carrier is present in great proportion. In that eventtribasic propellant charge powders are prepared with the use of anorganic solvent as a gelatinizing aid. The solvent, such as a mixture ofacetone and alcohol is added while the pulverous raw stock is beingmixed and is withdrawn in the end from the finished powder.

The usual procedure is to produce tribasic propellant charge powders inbatches. All the components of the powder, with the solvent being added,are mixed into a powder mass, which is moist with solvent, and kneadedin a closed kneader until the powder mass is homogenized andgelatinized. The gelatinized powder mass is compressed in a pot-typepress to yield powder strands which subsequently are cut to length andfrom which the solvent is removed by drying. This procedure is verylabor-intensive as the powder mixture requires a lot of manualtreatment.

Likewise known is a method which can be operated continuously to producetribasic propellant charge powders with solvents (German patent 24 61646 and U.S. Pat. No. 4,051,207). With this procedure a mixture, moistwith solvent, of all the components of the propellant charge powder ispreplasticized in an open, continuously operating kneader, i.e. it ispartly gelatinized. As a rule, the substance passes the kneader severaltimes. Subsequently the preplasticized substance containing solvent issubjected to final gelatinizing in an extruder and converted intogranular consistency. As a rule, the extruder, too, is to be passedseveral times. Thereupon the granular material is compressed in anotherextruder to yield the desired powder strands which still have to be cutto length and dried. This process is highly expensive in respect of themechanical equipment needed and the process control which must becarried out in adjusting and monitoring process parameters.

It is an object of the invention to indicate an improved process of thekind defined initially adapted for continuous operation and to berealized with less mechanical expenditure than before, the operatingcycle being easily adjustable and controllable. It is another object ofthe invention to provide an apparatus suitable for carrying out theprocess in question.

Starting from the process of the kind mentioned initially, the problemis solved in accordance with the invention in that raw stock moist withwater and premixed of nitrocellulose moist with water and a blasting oilis gelatinized in a continuously operating open kneader, being dried atthe same time, the kneading process being adjusted such that the rawstock upon leaving the kneading apparatus has been converted into afully gelatinized intermediate product having a residual water contentof less than 3%. When leaving the kneading apparatus the intermediateproduct is granulated, the granulated intermediate product (granularmaterial) and the third crystalline energy carrier as well as thesolvent are supplied to a continuously operating closed extruder inwhich they are homogenized by kneading and from which they are extrudedas powder strands which are moist with solvent. These powder strandsmoist with solvent are cut to length and dried.

Blasting or explosive oil in the context of the instant application isto be understood as being the usual liquid explosives which are made useof in the preparation of propellant charge powders, includingparticularly nitroglycerine and diglycol dinitrate and mixtures thereof.

The third crystalline energy carrier normally is nitroguanidine (Nigu;NQ). Likewise suitable are hexogene (RDX) or octogene (HMX) ornitropenta (PETN) as well as other crystalline energy carriers andmixtures of the same.

The additives are those which are customary in the preparation ofpowders, especially plasticizers and stabilizers. Their share by weightin the powder mixture is relatively small.

Drying of the powder strands refers to any kind of measure which iscustomary in the production of powders to remove the solvent practicallyentirely from the powder strands.

The method of the invention is characterized in that not all of thecomponents of the tribasic propellant charge powder are mixed togetherfrom the very beginning. At first, rather, a fully gelatinizedintermediate product is made of nitrocellulose and blasting oil alonewhich are premixed to present raw stock which is moist with water. Thisis accomplished in a single pass through an open kneading apparatus,without the use of solvents. The gelatinized and, therefore, dryintermediate product is granulated, and the granular material issupplied together with the third crystalline energy carrier to a closedextruder. That is when the solvent is added too. The kneading action inthe closed extruder converts the material into a homogeneous doughy massin wich the crystalline energy carrier is finely distributed uniformly.This mass which is moist with solvent is extruded out of the sameextruder, from the kneading, into powder strands which then are cut tolength as usual and dried.

The method according to the invention on the whole can be carried outcontinuously. Essentially only two machines are needed to practice it,namely the open kneading apparatus and the closed extruder, plus agranulating means downstream of the open kneader and means for conveyingthe granular material to the extruder. Process control is easy. Eachmachine needs to be passed but once. Even a frequent change of theformulations causes no problems in resetting and harmonizing therespective process parameters and in seeing to it that they aremaintained. It is advantageous in this context that the first part ofthe method, the treatment of the raw stock in the open kneader, hardlytakes a different course from the usual production of dibasic propellantcharge powders so that in this respect the routine and experience ofthat production can be relied on. Moreover, this assures a high degreeof safety. The risk of spontaneous ignition of the powder mixture in anyproduction process is particularly high until the nitrocellulose hasbecome fully gelatinized by the blasting oil for example, becauseblasting oil is separated from the raw stock. During that phase,however, the method according to the invention is carried out only inthe open kneading apparatus where the consequences of any spontaneouscombustion are much more harmless than in a closed processing machine,such as an extruder. In the extruder, on the other hand, the risk ofexplosion is greatly reduced with the method according to the inventionbecause of the addition of the solvent. Finally, the method according tothe invention can do with a much smaller quantity of solvent, based onthe production quantity, than the customary production process.Consequently, also the loss of solvent in drying the extruded powderstrands is less, and that makes the proces more economical andcontributes to the protection of the environment. For the same reason,the drying expenditure is much less in comparison with the conventionalmethod.

The open kneading apparatus used preferably is a shearing mill which isknown as such for homogenizing and plasticizing non-explosivethermoplastic substances, such as plastics (EP-A 0 148 966 and U.S. Pat.No. 4,605,309). Reference is made specifically to those two publicationsfor any further details of such a shearing mill. The shearing millcomprises two rollers which are adapted to be driven independently ofeach other and between which an adjustable roller nip is defined. Therollers are formed with spiralling grooves which assure that thematerial being processed is fed reliably into the nip and which, at thesame time, generate axial conveying movement in the roller nip from oneend of the roller pair to the other. With the method of the inventionthe raw stock moist with water is fed continuously to the two rollers ofthe simultaneous rolling mill at one end thereof in order then to beconveyed gradually to the other end of the two rollers while beingkneaded and subjected to intensive shearing. The raw stock which arrivesat the other end then is fully gelatinized. To permit such gelatinizing,the water must be removed from the raw stock. That is accomplished bysqueezing in the roller nip and also by evaporation of water since therollers of the shearing mill are heated. The raw stock supplied to theshearing mill preferably has a water moisture content of 20% to 30%. Theremoval of that water and the corrresponding progressing gelatinizing ofthe raw stock takes place gradually as the raw stock travels along therollers. Adjustment of the roller nip and of the numbers of revolutionof the two rollers permits process control such that the raw stock atthe discharge end of the roller pair is practically dry and, therefore,fully gelatinized. This means that the residual water level is less than3%, preferably less than 1%, being e.g. 0.5%. The fully gelatinized rawstock is located on one of the two rollers from which it may be removedcontinuously, while being granulated at the same time, for instance by agranulating head.

The method according to the invention also has the advantage thatformulations can be observed very strictly so that the production ofdifferent propellant charge powders is exactly reproducible since boththe intermediate product, because of its granulation, and the thirdcrystalline energy carrier can be supplied to the extruder in accuratelymetered quantities, such as by means of suitable dosing devices. Thereare various possibilites of feeding the extruder. If, in workingdirection of the extruder, first the granular material and thecrystalline energy carrier and subsequently the solvent are added, thenecessary extruder can be of especially simple design as only one commonopening is needed for the granular material plus crystalline energycarrier. The formation of bridges and plugging in the input opening forthe granular and energy carrier materials are avoided because thesolvent is added downstream of the feeding location of those materials.It proved that the homogeneity of the powder strand is even better iffirst the granular material, then the solvent, and thereafter thecrystalline energy carrier are added in the working direction of theextruder. For that purpose the extruder must be furnished with a firstfeeding opening in working direction for introduction of the granularmaterial, a subsequent opening for supply of the solvent, and finally anopening for supply of the crystalline energy carrier under pressure, anarrangement which is somewhat more complicated technically.

The additives may be supplied together with the premixed raw stock moistwith water to the kneading apparatus. Alternatively, the additives maybe supplied together with the granular material and/or the crystallineenergy carrier to the extruder. In specific cases, depending on theformulation, for instance, it may be convenient to make use of bothpossibilities in combination, such as by feeding certain additives tothe kneader and the remainder to the extruder.

The raw stock supplied to the simultaneous rolling mill usuallycomprises from 40% to 60%, preferably from 45% to 55% of nitrocellulose.The crystalline energy carrier is supplied to the extruder in an amountof from 10% to 55%, preferably from 40% to 55%, based on the granularmaterial fed previously.

The quantity of solvent supplied to the extruder conveniently rangesfrom 60 to 130 grams per kilogram of solids supplied (granularmaterial+crystalline energy carrier). This means that from 30% to 60% ofsolvent are saved as compared to conventional manufacturing methods.Suitable solvents among others are alcohol or acetone. The choicedepends among others on the degree of nitration of the nitrocelluloseemployed. The higher the degree of nitration, the more solvent generallyis needed or the stronger the solvent must be.

An apparatus for carrying out the method according to the invention ischaracterized by an open shearing mill, a downstream granulating means,a conveying means, and a closed extruder.

The invention will be described further, by way of example, withreference to the accompanying drawing, in which the only FIGURE is adiagrammatic presentation of an apparatus for preparing tribasicpropellant charge powders.

The apparatus illustrated comprises an open shearing mill 1, as known,for example from U.S. Pat. No. 4,605,309. The shearing mill comprisestwo rollers which are adpated to be driven independently of each otherand between which a roller nip or gap is defined. Pulverous raw stockmoist with water and premixed of nitrocellulose moist with water andblasting oil is supplied to the simultaneous rolling mill at one end ofthe two rollers. This supply is indicated in the drawing by a funnel andan arrow. In the shearing mill the raw stock is kneaded, thereby beinggelatinized with the water being removed. During processing, the rawstock travels toward the other end of the two rollers, and the raw stockarriving at the other end is fully gelatinized. A granulating means 2 isassociated with this end of the two rollers. It removes the gelatinizedraw stock or intermediate product from one of the two rollers andgranulates it at the same time. A conveying means 3 carries thegranulated intermediate product (granular material) to the feedingopening 5 of a closed extruder 4. The third crystalline energy carrieris fed into the same feeding opening 5. The granular material and thethird energy carrier conveniently each are introduced through a meteringdevice, not shown. Behind the feeding opening 5, in flow direction, theextruder 4 is furnished with an adding means 6 for entry of the solvent.Alternatively, another feeding opening 7 may be provided downstream ofthe adding means 6 for feeding the crystalline energy carrier separatelyfrom the granular material to the extruder. If so, that must be doneunder pressure because pressure already has built up in the interior ofthe extruder in the area of the feeding opening 7.

The extruder 4 is provided in the usual manner with one or two screws 8which are supported for rotation in the housing of the extruder andindicated only diagrammatically in the drawing. A die (not shown) isarranged at the outlet area 9 of the extruder. The material is extrudedthrough this die to form powder strands. The kneading action inside theextruder turns the granular material and the third energy carriertogether with the solvent, e.g. alcohol, into a doughy mass in which thethird crystalline energy carrier is finely distributed homogeneously.This mass which is moist with solvent is extruded at the end through thedie, e.g. into a 7-hole powder strand. The powder strands exiting fromthe extruder are cut to length in conventional manner (not shown), andthe solvent is removed from the same in suitable manner, e.g. in adrier.

The two rollers of the shearing mill are heatable to temperaturesbetween 80° C. and 120° C. The adjustable range of the rotational speedof the two rollers lies between 40 and 70 r.p.m. The gap width or nipbetween the two rollers as a rule is set to less than 2 mm. Thegranulating means 2 yields granular material of a size from 3 to 6 mm.The extruder 4 typically is of such design that its length to diameterratio is from 20 to 24. The temperature inside the extruder is between20° C. and 50° C. The number of revolutions of the screw is adjustablebetween 30 and 80 r.p.m.

To prepare a certain tribasic propellant charge powder, premixed rawstock consisting of 46% of nitrocellulose (N₂ =13,1%), 46% ofnitroglycerine, 8% of centralite I, and 0.7% of cryolite is processed inthe shearing mill 1 at a temperature of the one roller thereof of 90° C.and a rotational speed of 68 r.p.m. The temperature of the other rolleris 80° C. and its rotational speed 58 r.p.m. The mean throughput of theshearing mill is 52 kilograms per hour of material which is moist withwater. The raw stock when fully gelatinized, i.e. the intermediateproduct at the granulating means 2 has a content of water of less than0.5%. The dry granular material and nitroguanidine, as the third energycarrier, are metered into the extruder at a weight ratio of 45% ofgranular material and 55% of nitroguanidine. In the extruder, thematerial is homogenized with the aid of solvent and extruded to form a7-hole powder. The throughput is 40 kilograms per hour at a rotationalspeed of the screw of 60 r.p.m. and an extruder head pressure of 23 bar.

To prepare another tribasic propellant charge powder, premixed raw stockconsisting of 62% of nitrocellulose, 37% of nitroglycerine, 0.3% ofcentralite, 0.5% of arcadite, 0.1% of magnesium oxide, and 0.1% ofgraphite having an initial water content of 28% is processed in theshearing mill 1. The temperature of the first roller is 110° C., itsrotational speed 50 r.p.m. The temperature of the second roller is 90°C. and its rotational speed 45 r.p.m. The resulting throughput is 42kilograms per hour of dry mass at a water level of approximately 0.7% atthe granulating means. The granular material and nitroguanidine aremetered into the extruder at a weight ratio of 60%:40% . In the extruderthey are treated with acetone, as the solvent, in an amount of 3 litersper hour. The material is extruded to yield a 7-hole bulk powder. Therotational speed of the screw is 40 r.p.m., the head pressure is 27 bar,and the head temperature is 48° C. The resulting throughput is 30kilograms per hour.

what is claimed is:
 1. A method of preparing a tribasic propellantcharge powder comprising nitrocellulose, blasting oil, and a thirdcrystalline energy carrier, said method using a solvent and beingcharacterized by the following steps:(a) gelatinizing a raw stockcomprising a premixed paste of 40 to 60% nitrocellulose, a blasting oil,and a water moisture content of 20 to 30% is a continuously working,open kneading apparatus, said raw stock being dried simultaneously, theraw stock, upon leaving the kneading apparatus, being converted into afully gelatinized intermediate product having a residual water contentof less than 3%; (b) granulating the intermediate product as it leavesthe kneading apparatus; (c) supplying the granulated intermediateproduct, a third crystalline energy carrier and a solvent to acontinuously working, closed extruder in which they are homogenized bykneading and extruded in the form of powder strands moist with solvent;(d) cutting the powder strands moist with solvent to length and dryingthe same.
 2. The method as claimed in claim 1, characterized in that thekneading apparatus is an open shearing mill.
 3. The method as claimed inclaim 1, characterized in that the granulated intermediate product andthe crystalline energy carrier are first added together to the extruder,in an operating direction thereof, after which the solvent is added. 4.The method as claimed in claim 1, characterized in that the granulatedintermediate product, the solvent, and the crystalline energy carrierare added consecutively in an operating direction of the extruder. 5.The method as claimed in claim 11, characterized in that additives aresupplied to the kneading apparatus together with the premixed raw stock.6. The method as claimed in claim 1, characterized in that additives aresupplied to the extruder together with the granulated intermediateproduct material and/or the crystalline energy carrier.
 7. The method asclaimed in claim 1, characterized in that crystalline energy carrier issupplied to the extruder in an amount of from 10% to 55% of thegranulated intermediate product supplied to the extruder.
 8. The methodas claimed in claim 1, characterized in that the solvent is supplied tothe extruder in an amount of from 60 to 130 grams per kilogram of thegranulated intermediate product and the crystalline energy carriercombined supplied to the extruder.
 9. The method as claimed in claim 1,characterized in the raw stock is free of the third crystalline energycarrier and the solvent.
 10. An apparatus for carrying out the method asclaimed in claim 1, characterized by comprising, in the followingoperating order, an open shearing mill, a subsequent granulating means,a conveying means, and a closed extruder.
 11. The apparatus as claimedin claim 10, characterized in that, in its operating direction, theextruder has a first feed opening for the granulated intermediateproduct, an opening for supply in the solvent, and finally an openingfor supplying the crystalline energy carrier under pressure.